The dose rate measured at the centre of a tunnel excavated in an ore at 1 p. thousand of uranium was assessed to average 0.5 mrad/h. This value was verified by calculations talcing account of the source geometries and the main emitters of the uranium family. The dose rate from external exposure was calculated for two typical cases : — in a cylindrical tunnel (length = 50 m, diameter : 2 m) ; — in an open quarry 50 m long, with a vertical wall, 2 m high, and a platform. 8 m wide. The dose rates were calculated close to the walls, taking account of the various γ and β radiation emitted either by the walls or by atmospheric radon or by the ore dust deposited on the skin. The calculated values were close to the measured values. The radiation contribution depends on the conditions of exposure; exposure is mainly due to γ radiation close to very thick walls whereas β radiation is prevailing in a contaminated atmosphere or in the case of skin contamination.

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